1. Field of the Invention
This invention pertains to the field of cathodoluminescent phosphors used in display technologies.
2. Description of Related Art
Displays are the visual interface between users and the information that the users seek. Flat panel displays are thin and lightweight. Some commercial applications of flat panel displays include laptop computers, avionic displays, automobile dashboards, navigation displays, video phones, medical systems, pocket notepads, and miniature displays. Defense applications include wall-size command control displays, avionic displays, navigational displays, and head-mounted displays for soldiers.
The desired flat panel displays are lightweight, have good brightness, have sharp contrast, colors, and have wide viewing angel. For example, displays used by soldiers on the battlefield should be able to withstand harsh environmental conditions, should be lightweight, should provide a wide viewing angle, should provide viewing in bright light, and have high resolution.
Various display technologies exist or are under development for flat panel displays. These light emitting displays include active matrix liquid crystal displays, electroluminescent displays, plasma displays, and field emission displays. Each technology has its own merits and demerits and finds applications in various niche areas.
Flat panel field emission displays contain millions of micro-sized field emitters arranged in a matrix. These field emitters are addressed, in a matrix address, a pixel row at a time. The emitted electrons are accelerated toward the pixels on a screen a few millimeters away by an accelerating voltage. Each pixel is addressed by a large number of field emitters. An individual pixel consists of red, green, and blue sub-pixels. Based on the desired color from an individual pixel, the corresponding sub-pixel is addressed and its phosphor is excited producing its characteristic color.
Phosphor selection and their requirements vary based on the conditions of their use. Field emission displays work under relatively low voltage of about 500 V–10 kV and relatively high current density of about 50–100 μA/cm2 in contrast to cathode ray tubes that operate at high voltage of about 15,000–30,000 volts and low current density. At the low accelerating voltages used in field emission displays, slow impinging electrons do not penetrate very deeply into bulk of the phosphor, further increasing the current density at the phosphor surface. If the phosphor surface is resistive, this high current density can lead to serious charging, local heating, and thermal breakdown.
Most of the commonly used phosphors in cathode ray tubes, field emission displays and electroluminescent displays are sulfides that have highly resistive surfaces and typical particle sizes in the range of 1–10 microns. Some commonly used cathode ray tube phosphors are ZnS:Ag, Cl (blue), ZnS:Cu (green), Y2O3:Eu (red), and Y2O3S Eu (red) which are now being modified for field emission display applications. Some of the newly developed electroluminescent display phosphors, that maybe used in field emission display devices, are also sulfides: CaGa2S4:Ce (blue), SrGa2S4:Ce (blue), ZnS:Tb (green) SrGa2S4:Eu (green), CaS:Eu(red), (Ca, Sr)Ga2S4:Ce, (Ca, Sr)Ga2S4, and mixtures thereof. Under high coulomb charging, the surface temperature of resistive phosphors increases thereby resulting in dissociation and surface degradation (aging). Sulfur dioxide and hydrogen sulfide gases evolve from the phosphor surface and can damage the field emitters. Since field emission displays are addressed a line at a time, a given pixel is addressed for microseconds as opposed to nanoseconds in the case of cathode ray tubes. This long address time associated with the high current densities used in field emission displays make the conditions worse and result in severe current saturation and phosphor degradation. In many cases, aging is accelerated by heat that is associated with phosphor charging. Since the efficiency of the phosphor decreases as the accelerating voltage is decreased, the current density is increased to maintain brightness. The efficiency is further lowered by the high current densities involved in the process, surface charging and aging. Hence, the phosphors currently used in field emission displays have very poor efficiency.
Improvements in the efficiency of existing phosphors can be realized by reducing their resistivity, controlling their grain size, and modifying the surface chemistry of the phosphor particles. The efficiency of a phosphor can also be increased by using the quantum confinement effect. This is achieved by using quantum dots, i.e., <10 nm sized particles. However, the nanocrystalline quantum dot phosphors have very large surface areas that result in electron traps from impurities at the surface. The traps reduce the observed efficiency of the nanophosphors. Each individual nanocrystalline quantum dot must be isolated or prevented from agglomeration to observe quantum effect. All the problems, such as surface degradation, environmental effects, and aging, discussed in the case of microcrystalline or large phosphors, still exist in the case of nanocrystalline phosphors. These problems can be solved by using appropriate protective coatings on the phosphors used in electroluminescent displays, cathodoluminescent displays, and field emission display devices.
The Peterson U.S. Pat. No. 5,747,100 discloses a phosphor designed to emit radiation in the visible range when indirectly excited by low energy electrons, i.e. electrons which have energy in the tens of volts. Petersen's phosphor particle consists of a light-emitting particle with a UV-emitting coating. Petersen's light emitting particles are made from well known UV-excitable light emitting phosphors, which are known to emit visible light upon UV excitation and the coating is a well known UV-emitting material. The purpose of this coating is to convert the low voltage electrons into UV radiation and thereby exciting the UV phosphor particles. UV light is emitted in the spectrum of about 50–400 nm whereas visible light is emitted in the spectrum of about 400–700 nm.
For present field emission display commercial applications, phosphors operating under accelerating voltages of 100–10,000 volts, must last in excess of 10,000 hours of continuous operation without losing 50% of its original brightness. Presently, the phosphors do not meet this standard. The invention disclosed herein makes it possible for such phosphors to meet the standard.